The transmitter-receiver system according to the invention was in fact developed for remote control of the central locking system of a motor vehicle and consequently concerns a locking system actuated electronically or optically by means of coded signals. However, in principle it is additionally suitable for remote controls of whatever kind in which a control code (generally as difficult to copy as possible) is transmitted from a transmitter to a receiver. Thus, the invention is also suitable for the actuation of, for example (garage) doors and other closing and opening mechanisms, but also for example for the remote-controlled starting, stopping, switching, igniting, steering etc. of, in principle, any objects.
At the same time, the invention relates to a problem in initializing and re-initializing the transmitter-receiver system, as soon as it is namely intended to establish finally the code or a corresponding set of codes permitting actuation of the mechanism. When so doing, the code or the set of codes established is in each case to be sufficiently complicated and as difficult as possible for unauthorized persons to reproduce.
The invention namely relates to a further development of the specific transmitter-receiver system which is already known per se, in particular from DE-A1-32 25 754.
In the case of such a known system, an original code for the first-time initialization and also later in each case for a (renewed) re-initialization is generated and stored by the user of the system or by an authorized expert in a largely automatically controlled dialog between the transmitter and the receiver.
However, the initialization and re-initialization of the known transmitter-receiver system based on such a dialog requires relatively elaborate design measures and relatively complicated control processes.
In addition, a similar system is known from EP-A-0 292 217, (corresponding to U.S. Pat. No. 4,881,148), in which system the transmitter namely contains for preparation of the new code used in future by itself a random generator, which generates a random number. In addition, the transmitter transmits to the receiver in a unidirectional way, that is to say without a dialog taking place automatically and bidirectionally between the transmitter and the receiver, an original code, which for its part accordingly establishes the starting code, that is to say the new code, this transmitted original code being stored in the transmitter memory or in the receiver memory.
Other measures for (re-)initializing are likewise known or conceivable:
Associated sets of receivers (for example keys) and receivers with appropriately fixed preprogramming can be supplied by the manufacturer, which however requires careful stock keeping, and in general always the purchase of a complete set, even if for example only one transmitter or only one receiver is to be exchanged owing to defects.
Similar sets can also be produced which are not initialized until later by the authorized motor vehicle dealer by subsequent programming in an inconvenient way, namely by means of special devices.
In the case of a system with a fixed, unchanging code, the starting code represents the fixed code used all the time by the authorized person for actuation of the mechanism. In the case of a system with a changing code, however, the code representing authorization is constantly changed during the course of operation of the transmitter-receiver system on the basis of an algorithm which is also stored, so that then the original code establishes in particular that starting code which represents the first code which can be used by an authorized user for actuation of the mechanism. Subsequent authorized codes are occasionally called continuation codes. A changing code with constantly new, never recurring continuation codes offers much greater protection against misuse of the object, for example the motor vehicle, than a fixed code.